(Auteur) A large number of continuously operating reference station (CORS) networks have been established around the world to support various high-precision navigation and positioning applications. However, the presence of significant tropospheric delays makes rapid ambiguity recovery for long inter-station baselines of network real-time kinematic (RTK) systems a major challenge. Since tropospheric delays are strongly temporally correlated over short periods, we propose an undifferenced (UD) zenith tropospheric prediction model to effectively correct tropospheric errors on the subsequent epoch measurements. Using 2-h sessions of the independent baselines in a CORS network, the ambiguities are easily and reliably resolved with the conventional ionospheric-free combination method. The derived double-differenced (DD), ionospheric-free residuals are then converted to UD residuals for each satellite and all stations. The UD residuals and the corresponding wet coefficients of each satellite are used to construct the zenith tropospheric model. The model is reconstructed every 5 min for each station. The slant tropospheric errors of observations within this period can be predicted using the established models. Seven independent baselines with an average length of 97 km are used to test the ambiguity recovery performance of the proposed method. The experimental results show that the proposed tropospheric prediction model can efficiently reduce the effects of slant tropospheric errors and improve the float solution of ambiguities. The average initialization time with the proposed method is less than 111.5 s, which is a 45% improvement with respect to the conventional approach. The proposed method was shown to be effective for fast ambiguity recovery of long-range baselines between reference stations.

(Auteur) Thanks to the international GNSS service (IGS), which has provided an open-access real-time service (RTS) since 2013, real-time precise point positioning (RT-PPP) has become a major topic in the time community. Currently, a few scholars have studied RT-PPP time transfer, and the correlation of the receiver clock offsets between adjacent epochs have not been considered. We present a receiver clock offset model that considers the correlation of the receiver clock offsets between adjacent epochs using an a priori value. The clock offset is estimated using a between-epoch constraint model rather than a white noise model. This approach is based on two steps. First, the a priori noise variance is based on the Allan variance of the receiver clock offset derived from GPS PPP solutions with IGS final products. Second, by applying the between-epoch constraint model, the RT-PPP time transfer is achieved. Our numerical analyses clarify how the approach performs for RT-PPP time and frequency transfer. Based on five commonly used RTS products and six IGS stations, two conclusions are obtained straightforwardly. First, all RT-PPP solutions with different real-time products are capable of time transfer. The standard deviation (STD) values of the clock difference between the PPP solutions with respect to the IGS final clock products are less than 0.3 ns. Second, the STD values are reduced significantly by applying our approach. The reduction percent of STD values ranges from 4.0 to 35.5%. Moreover, the largest improvement ratio of frequency stability is 12 as compared to the solution of the white noise model. Note that the receiver clock offset from IGS final clock products is regarded as a reference.

(Auteur) The rapid development of the BeiDou Satellite Navigation System (BDS) and other Global Navigation Satellite System (multi-GNSS) constellations provides a great opportunity to contribute to earthquake early warning systems in terms of capturing displacement and velocity waveforms for the estimation of magnitude and fault slip inversion. In this study, we demonstrate the capability of BDS and the benefit of multi-GNSS for real-time capturing seismic waveforms using the combined high-rate BDS + GPS + GLONASS data collected during the 2017 Mw 6.5 Jiuzhaigou earthquake. For this event, we found that the displacements, derived from BDS precise point positioning (PPP) are better than that of Global Positioning System-only (GPS) results, especially in the east and vertical components with improvements of 43% and 23%. While the velocity waveforms from BDS present a comparable performance with GPS. the multi-GNSS fusion can significantly improve the accuracy by 47%, 55%, and 28% in the east, north, and vertical components compared with GPS-only results. The BDS and multi-GNSS derived displacement waveforms agree quite well with those obtained from integrating the acceleration, with accuracy at the millimeter level. In addition, the theoretical permanent displacement field calculated from a finite-fault slip model is selected as an independent reference, and the differences between GNSS derived permanent displacements and theoretical permanent displacements are mostly less than 1 mm. Therefore, we conclude that the BDS and multi-GNSS fusion can significantly contribute to the real-time capture of accurate seismic waveforms and that it has the potential to benefit for earthquake early warning and rapid geohazard assessment.

(Auteur) Real-time single-frequency precise point positioning (RT-SF-PPP) has become a desired positioning approach because it can achieve high positioning accuracy with a low-cost global navigation satellite system (GNSS) chipset or receiver. For single-frequency precise point positioning (SF-PPP) applications, the ionospheric delay is a dominant error source, and thus the quality of applied ionospheric products is critical to the performance of SF-PPP. To meet the demands of the RT-SF-PPP users, the international GNSS service (IGS) is planning to provide open-access real-time ionospheric products. By now, the Centre National d’Études Spatiales (CNES) is the only IGS analysis center (AC) to broadcast real-time ionospheric vertical total electron content (VTEC) message through its real-time service (RTS). The quality of the CNES real-time ionospheric products is drawing increasing attention from the GNSS community. We evaluate the quality of CNES real-time VTEC message both in the ionospheric correction domain and positioning domain. First, 374 consecutive days of CNES VTEC products are collected and compared with the IGS final global ionospheric map (GIM) products. Second, slant total electron content (STEC) computed with CNES VTEC message is fully assessed with respect to STEC derived from dual-frequency GNSS measurements. Finally, RT-SF-PPP is conducted for assessing the quality of CNES real-time ionospheric products in the positioning domain. The degree and order of the spherical harmonic expansions broadcasted in the CNES VTEC messages changed from 6 to 12 in the time span of collected data, the effects of higher degree and order parameters are investigated at the same time in the experiments above.

(Auteur) Previous studies have proved that commercial, off-the-shelf, geodetic-quality Global Positioning System (GPS) receivers can monitor water level using multipath interferometric characteristics from the sea surface. A Lomb–Scargle periodogram (LSP) is typically used to extract the multipath frequency from the signal-to-noise ratio (SNR), which relates to the vertical distance between the antenna phase center and the reflecting surface, and subsequently, estimate sea levels. A wavelet transform can be used to analyze the time series with a nonstationary power at different frequencies, as can be the case for multipath signals. In addition, wavelet transform analysis estimates the local energy and instantaneous frequency of the signal at every epoch, which is beneficial for water-level estimation. Therefore, we attempt to extract the instantaneous multipath frequency using wavelet transform analysis for water leveling with a geodetic GPS receiver. This study analyzed SNR data of GPS L1C from three test sites: the Kachemak Bay site PBAY (AK, USA), the Friday Harbor site SC02 (WA, USA), and the Brest Harbor site BRST (France). These sites are located in different multipath environments, from a rural coastal area to a busy harbor, and they experience different tidal regimes. The results show that the wavelet analysis has potential for the retrieval of sea level heights. For measurement site PBAY the comparison of the results obtained by the wavelet and LSP analysis yield consistency within a few percent. At the other two sites, SC02 and BRST, the agreement is significantly lower.

(Auteur) Today Global Navigation Satellite Systems (GNSS) tropospheric products, such as zenith total delays (ZTD) and zenith wet delays (ZWD), are widely used as complementary data sets in numerical weather prediction models. In particular, the wet delays are treated as unknown parameters in GNSS processing and are estimated with other parameters such as station coordinates. In this study, we investigate the effects of Phase Center Correction (PCC) models on ZWD, integrated water vapor (IWV) and horizontal gradients derived from Global Positioning System (GPS) observations. Two solutions were generated using the GAMIT software over the European Reference Frame (EUREF) Permanent GNSS Network (EPN). The first (reference) solution was derived by applying the International GNSS Service (IGS) type-mean PCC models, while for the second solution PCC models from individual calibrations were used. The solutions were generated identically, except for the PCC model differences. The tropospheric products from the two solutions were then compared, with the assumption that common signals would be differenced out. The comparison of the two solutions clearly shows a bias in all tropospheric products, which can be attributed to PCC model deficiencies. Overall, mean biases of 1.8, 0.3, 0.14 and 0.19 mm are evident in ZWD, IWV, North–South and East–West gradients, respectively. Moreover, the differences between the two solutions show seasonal variations. For all antenna types, the ZWD and IWV differences are dominated by white plus power-law noise, with the latter characterizing the low-frequency spectrum. On the other hand, the horizontal gradients exhibit a white plus first-order autoregressive noise characteristic with less than 1% white noise. The individual PCC model provides a better fit to an external independent model in terms of gradient estimates and also provides up to 3% more carrier phase integer ambiguity resolution.

(Auteur) Due to the rapid deployment of the Galileo constellation, Galileo is now able to contribute to GNSS precise point positioning (PPP) ambiguity resolution (AR) with 17 operational satellites as of December 2017. We estimate the satellite fractional cycle bias (FCB) based on globally distributed MGEX stations and assess the Galileo FCB quality by a comparison with that of GPS and BDS. Results of 60 days indicate that the quality of Galileo wide-lane (WL) FCB is better than GPS and BDS in terms of data usage rate, residual distribution, as well as standard deviation of daily estimates. The RMS of Galileo WL FCB residuals is 0.071 cycles, while that of GPS and BDS are 0.089 and 0.117 cycles, respectively. The standard deviation of Galileo daily WL FCB is 0.010 cycles, while that of GPS and BDS is 0.018 and 0.043 cycles. We attribute the better quality of Galileo WL FCB to its signal modulation, AltBOC, which significantly compresses the multipath effect for pseudorange measurement. Within the Galileo constellation, the performance of In-Orbit Validation (IOV) satellites WL FCB is worse than that of Full Operational Capability (FOC) satellites as a result of a reduction in the power of the transmitted signal. The performance of the two highly eccentric satellites is comparable to other FOC satellites. The overall quality of Galileo narrow-lane (NL) FCB is slightly worse than that of GPS but better than that of BDS. The RMS of Galileo NL FCB residuals is 0.062 cycles, while that for GPS and BDS is 0.050 and 0.086 cycles respectively. In addition, the NL FCB quality of FOC, IOV (except E19), as well as the two eccentric satellites, shows no significant difference in terms of data usage rates and residuals. Galileo PPP AR solutions are conducted at 20 MGEX stations with 3-h sessions for 10 days. The positional biases of AR solutions are 0.7, 0.6, and 2.1 cm for east, north and up components respectively, while those for float solutions are 2.1, 1.1, and 2.7 cm, corresponding to the improvements of 67, 45, and 22%, respectively. These results demonstrate that, currently, Galileo FCB can be estimated with accuracy comparable with GPS and BDS, and the Galileo observations can bring an obvious benefit to ambiguity-fixed PPP.